Adsorption Kinetics of Ultrathin Polymer Films in the Melt Probed by Dielectric Spectroscopy and Second-Harmonic Generation

Abstract

We studied the adsorption kinetics of supported ultrathin films of dye-labeled polystyrene (l-PS) by combining dielectric spectroscopy (DS) and the interface-specific nonlinear optical second harmonic generation (SHG) technique. While DS is sensitive to the fraction of mobile dye moieties (chromophores), the SHG signal probes their anisotropic orientation. Time-resolved measurements were performed above the glass transition temperature on two different sample geometries. In one configuration, the l-PS layer is placed in contact with the aluminum surface, while in the other one, the deposition is done on a strongly adsorbed layer of neat PS. From the time dependence of the dielectric strength and SHG signal of the l-PS layer in contact with the metal, we detected two different kinetics regimes. We interpret these regimes in terms of the interplay between adsorption and orientation of the adsorbing labeling moieties. At early times, dye moieties get adsorbed adopting an orientation parallel to the surface. When adsorption proceeds to completeness, the kinetics slows down and the dye moieties progressively orient normal to the surface. Conversely, when the layer of l-PS layer is deposited on the strongly adsorbed layer of neat PS, both the dielectric strength and the SHG signal do not show any variation with time. This means that no adsorption takes place.

Effects of surface modification and thermal annealing on the interfacial dynamics in core–shell nanocomposites based on silica and adsorbed PDMS

“…Adoption of various conformations by polymer chains (tails, loops, trains [31]) at the interfaces with solid surfaces were found also of importance for predicting interfacial polymer properties [3,[32][33][34]. Theodorou and coworkers [3,29] showed that polymer chains close to an attractive solid surface are pronouncedly flattened, or else more parallel to the surface, as compared to those away from the surface, this result coming in agreement with respective experimental studies [20,35]. Finally, Bitsanis and Brinke [32] and Harmandaris and coworkers [3,33,34] pointed to the polymer chain length as a crucial parameter for adoption of different conformations of polymers adsorbed on solid surfaces and of polymer confined between solid surfaces, respectively.…”

“…More specifically, according to recent studies on adsorbed polymers [6,20,21] there may exist two populations of chain segments at interfaces which can be considered responsible for the molecular mobility recorded in BDS as the interfacial a int process, i.e. (a) extended tails with bulk-like density but reduced mobility and cooperativity (Scheme 1a), and (b) flattened chain segments which form the inner quite dense region due to multiple contact points with the silica surface (loops, Scheme 1a).…”

“…According to the models described above, the loop-like conformed chains are weakly attached [20,21] on the surfaces and their concentration can increase as temperature increases, without change in interfacial polymer density, e.g. by simultaneous decrease of loops' maximum distance (height) from the adsorbing surface [21], similarly to polymers adsorbed onto solid surfaces during chemical [19] and thermal [20] annealing. On the other hand, in the case of higher tail/loop ratio (Schemes 1b and c, i.e.…”

“…Moreover, they proposed that the increased interfacial layer thickness in polydimethylsiloxa ne(PDMS)/titania as compared to PDMS/silica nanocomposites [8,9,18] originates from the stronger particle-polymer hydrogen bonding. Furthermore, we have recently demonstrated that the structure of adsorbed complexes of linear PDMS onto high specific surface area fumed silica particles (aerosils) depends strongly on the adsorption conditions and the subsequent thermal treatment [17], similarly to polymers adsorbed on solid surfaces [19][20][21].…”

“…Parallel to the study of the interfacial layer in polymer nanocomposites, various studies of polymers adsorbed thermally or/and chemically on solid surfaces [19,20,22,23], as well as of thin polymer films [24], have demonstrated similarities between polymer nanocomposites and thin polymer films in thermal stability, polymer chain conformations and dynamics. Core-shell based nanocomposites [25,26] form a next class of interest in the same direction, where the polymer is adsorbed in multiple layers (shells) onto the nanoparticles (core).…”